PART II.—THEORY AND EXAMPLES.
THE STRENGTH OF MATERIALS, STRUCTURES, AND
OUR intention in this chapter is to treat of the cohesive
strength of the materials used in Mechanical Engineering, of
practical testing to obtain strength constants, and of the use of
the latter in proportioning machine parts, so far as may be done.
Load is the total effect on the structure of the external
forces, and may be ' dead' or ' live,' concentrated or distributed
(seepp. 391 and 438).
Stress is the cohesive force within the material called into
play to resist the load. (See Appendix IIL, p. 920.)
Strain is the deformation produced by the stress.
Kinds of Stresses.—Only three simple stress-strain actions
are possible: tension (pulling), compression (thrusting), and shear
(cross-cutting). Bending is a mixed action, and local compression
produces a bearing stress. Fig. 320 shews the distortions and
fractures produced by these various stresses. «§
Elasticity is the property of regaining original shape and
dimensions after distortion ; very apparent in an elastic body,
but scarcely perceptible in a rigid one. In 1676, Hooke pro-
pounded the law * ut tensio sic vis' (as the tension, so this
strain), meaning that stress and strain are proportional, if within
the elastic limit of the material.
Limit of Elasticity.—A bar being subjected to an increas-
ing stress (of any kind), will receive also a proportionately increas-
ing strain (of the same kind) until the elastic limit is reached,
after which the strains increase more'rapidly than the stresses till
rupture occurs. Shewing this by a diagram, Fig. 321, o is an